What should I wear to my boyfriend's sister's wedding?

Wear a tea-length sleeveless or halter top dress ... I'd avoid the strapless, even tho' it's very "in" right now. Check out Lane Bryant - they've got some great spring dresses. Pair the dress with some strappy heels. :) And if you aren't sure what shoe style to wear - find the dress first, then take it to the Nordstrom shoe department & show it to them & get their opinion. I've done that and it was wonderful. :)